The correct answer is: A. At rest without any energy stored in it.
When the initial conditions of a system are specified to be zero, it implies that the system is at rest without any energy stored in it. This is because the initial conditions of a system determine its state at the beginning of a time interval. If the initial conditions are zero, then the system will be at rest and will not have any energy stored in it.
Option B is incorrect because it implies that the system is working normally with a reference input. However, if the initial conditions are zero, then the system will not be working normally because it will not have any energy stored in it.
Option C is incorrect because it implies that the system is working normally with zero reference input. However, if the initial conditions are zero, then the system will not be working normally because it will not have any energy stored in it.
Option D is incorrect because it implies that the system is at rest but stores energy. However, if the initial conditions are zero, then the system will not be at rest because it will not have any energy stored in it.